1. Field of the Invention
This invention relates to a system for detecting defects on an optical information recording medium, and more particularly, to a system of this kind which detects a defect on a recording surface of a recording medium by the use of a photoelectrically converted signal supplied by a quadrant-separated type light detecting element which receives return light beams obtained by irradiating the recording surface with a single reading light beam.
2. Description of the Related Background Art
In recent years, the DVD (digital video disk) system has been put to practical use and is coming into wide use. In carrying out tracking control of the DVD and other high-density information recording media, the main method is to detect a tracking error by using a single reading light beam (and return light beam(s) thereof).
In particular, a method of this kind is based on a tracking error detection technique called the phase difference-based detection method. Various forms of the above methods are employed.
Japanese patent kokai No. 4-10230 and Japanese patent publication Nos. 7-11867 and 8-34007 discloses tracking error detection methods based on the phase difference-based detection method.
However, one of disadvantages with the methods using a single reading light beam for detecting a tracking error is that the light beam is very sensitive to a defect on not only a surface or recording surface of a recording medium but also a defect on a surface of an inner layer as well as an improperly formed mark equivalent to such defects. Such defects are likely to cause degraded playability. In one of the phase difference-based detection methods, a photodetector is used which has four light-receiving surfaces (quadrant surfaces) divided by two dividing lines. It is noted that one dividing line extends in a direction along a track of an objective recording medium and that the other dividing line orthogonal to the one dividing line. The return light beams reflected from the recording medium and incident on the respective light-receiving surfaces are photoelectrically converted to generate respective electric signals. Then, a reference signal is obtained by adding up all the four electric signals to obtain one signal (called an "RF signal", an "HF signal", or a "sum total signal"). Each pair of signals supplied from two light-receiving surfaces symmetrical with respect to the point of intersection of the two dividing lines are added up to obtain a so-called diagonal sum signal, and the phases of the diagonal sum signals are detected with reference to the reference signal. Therefore, a tracking error signal is detected based on the detected phases of the diagonal sum signals.
In the phase difference-based detection method described above, if the reading light beam passes through a defect or scratch, any changes occur in the RF signal and the diagonal sum signals. Accordingly, a reading signal to be demodulated as an information signal is mixed with noises, and moreover, proper execution of tracking servo control is hindered due to absence of a correct tracking error signal, which results in degradation of playability. In such a case, however, if the defect is detected from the return light beams (i.e. electric signals generated from the received return light beams), it will be possible to recognize an occurrence of the noise in the reading signal and generation of an incorrect tracking error signal at the same time. Therefore, degradation of playability can be avoided by inhibiting reproduction of information and execution of tracking servo control based on the respective reading signal and incorrect tracking error signal generated in response to the defect. Therefore, accurate detection of a defect on a recording medium is crucial to a system which employs the method using a single reading light beam for tracking error detection.
On the other hand, when the reading light beam scans a track of the recording medium in a so-called off-track (or detrack) state, the states of the RF signal and the diagonal sum signals change similarly to the above case of the reading light beam passing through a defect. However, a tracking error signal generated in a pure off-track state of the reading light beam is correct, so that changes in the states of the RF signal and the diagonal sum signals are properly produced in dependence on the amount and polarity of the tracking error. Therefore, when defect detection is performed based on the output signals generated from received return light beams, i.e. the RF signal and the diagonal sum signals, it is required to clearly distinguish the detection of a defect from the detection of a tracking error.
Moreover, among defects which a recording medium may have thereon is a defect referred to as a "mirror pinhole", and it is required that such a defect be detected reliably.